Microelectronic features are typically formed in microfeature workpieces (including semiconductor wafers) by selectively removing material from the wafer and filling in the resulting openings with insulative, semiconductive and/or conductive materials. One typical process includes depositing a layer of radiation-sensitive photoresist material on the wafer, then positioning a patterned mask or reticle over the photoresist layer, and then exposing the masked photoresist layer to a selected radiation. The wafer is then exposed to a developer, such as an aqueous base or a solvent. In one case, the photoresist layer is initially generally soluble in the developer, and the portions of the photoresist layer exposed to the radiation through patterned openings in the mask change from being generally soluble to being generally resistant to the developer (e.g., so as to have low solubility). Alternatively, the photoresist layer can be initially generally insoluble in the developer, and the portions of the photoresist layer exposed to the radiation through the openings in the mask become more soluble. In either case, the portions of the photoresist layer that are resistant to the developer remain on the wafer, and the rest of the photoresist layer is removed by the developer to expose the wafer material below.
The wafer is then subjected to etching or metal deposition processes. In an etching process, the etchant removes exposed material, but not material protected beneath the remaining portions of the photoresist layer. Accordingly, the etchant creates a pattern of openings (such as grooves, channels, or holes) in the wafer material or in materials deposited on the wafer. These openings can be filled with insulative, conductive, or semiconductive materials to build layers of microelectronic features on the wafer. The wafer is then singulated to form individual chips, which can be incorporated into a wide variety of electronic products, such as computers and other consumer or industrial electronic devices.
As the size of the microelectronic features formed in the wafer decreases (for example, to reduce the size of the chips placed in electronic devices), the size of the features formed in the photoresist layer must also decrease. In some processes, the dimensions (referred to as critical dimensions) of selected features are evaluated as a diagnostic measure to determine whether the dimensions of other features comply with manufacturing specifications. Critical dimensions are accordingly selected to be the most likely to suffer from errors resulting from any of a number of aspects of the foregoing process. Such errors can include errors generated by the radiation source and/or the optics between the radiation source and the mask. The errors can also be generated by the mask, by differences between masks, and/or by errors in the etch process. The critical dimensions can also be affected by errors in processes occurring prior to or during the exposure/development process, and/or subsequent to the etching process, including variations in deposition processes, and/or variations in material removal processes, e.g., chemical-mechanical planarization processes.
One general approach to correcting lens aberrations in wafer optic systems (disclosed in U.S. Pat. No. 5,142,132 to McDonald et al.) is to reflect the incident radiation from a deformable mirror, which can be adjusted to correct for the aberrations in the lens optics. However, correcting lens aberrations will not generally be adequate to address the additional factors (described above) that can adversely affect critical dimensions. Accordingly, another approach to addressing some of the foregoing variations and errors is to interpose a gradient filter between the radiation source and the mask to spatially adjust the intensity of the radiation striking the wafer. Alternatively, a thin film or pellicle can be disposed over the mask to alter the intensity of light transmitted through the mask. In either case, the filter and/or the pellicle can account for variations between masks by decreasing the radiation intensity incident on one portion of the mask relative to the radiation intensity incident on another.
One drawback with the foregoing arrangement is that it may be difficult and/or time-consuming to change the gradient filter and/or the pellicle when the mask is changed. A further drawback is that the gradient filter and the pellicle cannot account for new errors and/or changes in the errors introduced into the system as the system ages or otherwise changes.
Another approach to addressing some of the foregoing drawbacks is to provide a pixilated, diffractive optical element (DOE) in place of a fixed geometry diffractive device. The pixilated DOE typically includes an array of electrically addressable pixels, each of which has an “on” state and a “off” state. Pixels in the on state transmit light and pixels in the off state do not. Accordingly, such DOEs can be repeatedly reprogrammed to generate new patterns, allowing the user to avoid the cost associated with generating a new diffractive element for each new microfeature workpiece design. However, a drawback with this approach is that toggling the pixels between two binary states may not provide adequate control over the radiation impinging on the microlithographic workpiece, which can in turn result in sub-standard or otherwise unacceptable workpieces.